1. Field of the Invention
This invention relates to a charged particle beam exposure technique, and particularly to a charged particle beam exposure technique for drawing fine patterns such as on ICs by using charged particle beams, e.g., electron beams.
2. Description of the Related Art
ICs have improved in integration and function in recent years, and continue to play a major role in technological advancement in the field of computers, communications, machine control, etc. ICs have achieved quadruple integration in the past two to three years, and DRAMs have rapidly advanced in integration from 1 Mb to 4 Mb, 16 Mb, and 64 Mb.
Such high integration is achievable only with advancements in fine processing. The current level fine processing has mainly been due to an optical exposure technique. Current optical exposure technique realize a precision of 0.5 .mu.m. It is said that the limit of the optical exposure technique lies around 0.3 .mu.m. It is difficult, therefore, to optically form a fine pattern of 0.15 .mu.m or below.
On the other hand, a charged particle beam exposure technique is applicable for a fine processing of 0.10 .mu.m or below at a positioning accuracy of 0.05 .mu.m or below.
However, conventional charged particle beam exposure techniques are not suitable for mass-producing LSIs because they achieve low throughput. New techniques such as a block exposure method and a blanking aperture array method proposed by the inventors of the present invention realize a throughput of about 1 cm.sup.2 /sec. With such throughput ensured, the charged particle beam exposure technique is more advantageous than any other lithography techniques in terms of fine processing, positioning accuracy, quick turnaround, reliability, and software improvement.
The charged particle beam exposure technique involves two deflectors for deflecting charged particle beams; an electrostatic deflector, and an electromagnetic deflector. Generally, the electrostatic deflector is speedier than the electromagnetic deflector.
A lens barrel for charged particle beams is sometimes designed to contain only the electrostatic deflectors to improve exposure speed. For high-speed exposure, it is also necessary to increase the intensity (current density) of a charged particle beam, e.g., an electron beam. To increase the current density as well as secure resolution of the beam, it is necessary to shorten the focal length and minimize the spherical and chromatic aberration of at least the last projection lens in the barrel.
A patterned beam of large current density contains many electrons which act to blur the beam itself because of Coulomb's force acting between the electrons. To reduce the blur, the lenses in the barrel must be short. When an electromagnetic lens is shortened with deflection being carried out only with the electrostatic deflectors, deflection efficiency drastically deteriorates. In particular, the throughput of step-and-repeat mode exposure drops below a usable level. Even continuous movement mode exposure will not provide a practical exposure speed if a deflection width is less than 1 mm and if a stage moving speed is less than 100 mm/s.
To improve deflection efficiency, the voltage of an electrostatic deflection amplifier may be increased. The high-output-voltage transistors, used for this purpose, however, are slow and frequently malfunction.
Even if these problems are solved, in order to use the electrostatic deflector as a main deflector, it may not be disposed above a subdeflector used for scanning subfields of an exposure object, because the subdeflector is also an electrostatic deflector. Instead, they must be arranged side by side.
Because of this, the lenses cannot be reduced beyond a certain length. This means that the blur because of Coulomb's force will not effectively be removed.
Accordingly, a practical electron beam exposure apparatus uses both an electromagnetic deflector and an electrostatic deflector.
The electromagnetic deflector comprises a plurality of coils disposed orthogonally to a beam axis and is capable of widely deflecting a beam even in a direction orthogonal to the beam axis. Since the electromagnetic deflector uses coils, it takes a long time for a current disturbed by inductance of the coils to settle.
In addition, a beam position is sometimes changed by an eddy current, and therefore, a time is needed for settling the beam position.
The inventors of the present invention have proposed a method of shortening the settling time required by the main deflector (U.S. Pat. No. 4,607,333).
A charged particle beam exposure apparatus according to this proposal will be roughly explained with reference to FIGS. 1 and 2.
FIG. 1 schematically shows the charged particle beam (electron beam (EB)) exposure apparatus.
A lens barrel includes a beam source 4 for emitting a charged particle (electron) beam 24. The beam 24 is widely deflected by a main deflector 7 composed of electromagnetic coils, and narrowly and quickly deflected by a subdeflector 8 composed of electrostatic deflector electrodes 8a and 8b. A shutter 11, such as a blanking electrode, is disposed across the axis of the beam 24 to optionally turn ON and OFF the beam 24. A wafer 1 to be exposed to the beam 24 is placed on a movable stage 10. According to a continuously moving method, the stage 10 is continuously moved in one direction.
Exposure pattern data are stored in a memory 14, and when required, read by a CPU 16 through a bus 15. Digital pattern data provided by the CPU 16 are processed by digital control circuits 17 and 20, converted into analog signals by digital-to-analog converters (DACs) 18a, 18b, and 21, and supplied to amplifiers 19a, 19b, and 23. The amplifier 23 is for the main deflector 7. An amplified signal from the amplifier 23 is further amplified by an amplifier 22 and supplied as a current I to the coils of the main deflector 7. The current I flowing through the deflector 7 produces, through a monitor resistor RM, a voltage V2 that is proportional to the current. Since the main deflector 7 has inductance L, it takes time for the current I to rise. Accordingly, during a transient period, the output voltage of the amplifier 23 changes differently from the output current I of the amplifier 22.
The output voltage of the amplifier 23 is amplified by an adjusting amplifier 29 to a voltage V1 corresponding to the output voltage V2 of the monitor resistor RM. The voltages V1 and V2 are supplied to a differential amplifier 25. In a steady state, the two voltages V1 and V2 are equal to each other.
The differential amplifier 25 generates an output voltage V3 according to the difference between the voltages V1 and V2. The output voltage V3, i.e., a differential signal is supplied to amplifiers 26 and 28 through resistors R1a and R1b. The differential signal to the amplifier 28 is inverted through an inverter 27.
The subdeflector amplifiers 19a and 19b supply voltages of opposite polarities to the amplifiers 26 and 28 through resistors R2a and R2b, respectively. The amplifiers 26 and 28 have feedback resistors R3a and R3b, respectively. The amplifiers 26 and 28 supply voltages to the electrodes 8a and 8b of the subdeflector 8 for scanning subfields with the beam 24.
According to U.S. Pat. No. 4,607,333, a pattern to be drawn on the wafer 1 is divided into subfields as shown in FIG. 2(A). Namely, a field 30 on the wafer 1 is divided into a plurality of long stripes 31 that extend in a stage moving direction indicated by an arrow mark P. Each of the stripes 31 is divided into a plurality of bands 32 that are orthogonal to the stage moving direction P. Further, each of the bands 32 is divided into subfields 33 that are scanned by the subdeflector 8.
To scan the subfields 33, the main deflector 7 deflects the beam 24 to the center of one of the subfields, and the high-speed subdeflector 8 deflects the beam 24 to draw patterns in the subfield. If the subdeflector 8 has a deflectable area of, for example, 100 .mu.m.times.100 .mu.m, the subfield 33 also has an area of 100 .mu.m.times.100 .mu.m.
To successively expose the stripes 31, the stage 10 is continuously moved in the direction P, and the beam 24 is successively deflected to the centers of the subfields 33 in each stripe 31. While the main deflector 7 is oriented toward the center of one subfield 33, the subdeflector I]deflects the beam 24 within the subfield 33 to draw patterns in the subfield 33. After the subfield 33 is exposed, the beam 24 is deflected by the main deflector 7 to the center of the next subfield 33, and then deflected by the subdeflector 8 to expose the subfield 33 in question.
When operating the main deflector 7, the output of the main deflector amplifier 23 rises differently from the current I flowing through the main deflector 7.
FIG. 2(B) shows changes in the voltage V1 that correspond to a rising output voltage of the DAC 21. The DAC 21 voltage and amplifiers 23 and 29 voltage rise quickly, so that a change in the voltage V1 corresponding to a movement of 100 .mu.m can be completed within 1 to 2 .mu.sec.
On the other hand, the current I flowing through the coils of the main deflector 7 needs a long time to rise because of the inductance L of the coils. FIG. 2(C) shows a rising waveform of the output voltage V2 of the monitor resistor RM corresponding to the coil current I. The voltage V2 rises relatively slowly, overshoots, and gradually settles. It takes about 100 .mu.sec for the coil current I to settle.
If a wait time of 100 .mu.sec is needed whenever the position of the beam 24 is deflected by the main deflector 7, a wait time of one second will be needed to expose an area of 10 mm square. If an actual exposure time is one second, two seconds in total will be needed.
To shorten this wait time, the differential amplifier 25 detects a difference between the output voltage of the DAC 21 and the monitor voltage of the coil current I, and provides a compensation voltage, i.e., a differential signal V3.
FIG. 2(D) shows a waveform of the differential signal V3. The differential signal V3 rises as the output of the DAC 21 rises, then gradually falls according to a rise of the monitor voltage of the coil current I, and settles.
Referring again to FIG. 1, the differential signal V3 is additionally applied to the subdeflector amplifiers 26 and 28. As a result, even if the coil current I is not yet stable, the differential signal V3 which settles relatively quickly compensates changes in magnetic fields, to thereby enable patterns to be drawn at a required accuracy. A time necessary for stabilizing the differential signal V3 at a compensation possible value is about 40 .mu.sec, which corresponds to about 10 .mu.m in terms of positional change. By feeding the differential signal V3 back to the subdeflector 8, the wait time of the main deflector 7 can be shortened from about 100 .mu.sec to about 40 .mu.sec.
Although this technique halves the wait time of the main deflector 7, the shorter the wait time, the better.